By switching from gasket seals of PTFE to custom gaskets of DuPont™ Kalrez® perfluoroelastomer parts in the sensor head of a process instrument refractometer used by the food, pulp and paper and chemical industries, process instrument manufacturer K-Patents Oy, Vantaa, Finland, was able to dramatically extend instrument service life, increase reliability and safety, and reduce costs for the company and its customers.
Aggressive environment Through permanent in-line fluids immersion, K-Patents’ “PR-01-S” refractometer is exposed to temperatures from –20 ° to +220 °C, pressures from –0,7 to +25 bar, and some 500 process fluids and chemicals, many of which are extremely aggressive. Delicate digital detector circuits and fiber optics in the sensor head are sealed by two gaskets from attack by aggressive fluids. Because of inherent inelasticity, the original PTFE gaskets could not withstand the dynamic temperature fluctuations of many food, pulp and paper and chemical manufacturing processes, creating a leak path allowing process media to enter and damage the device. As a result, costly replacement of the PTFE seal became necessary approximately every 6–12 months.
Below is the third and final section of the white paper, which will discuss the importance of proper seal and groove design.
Proper Seal & Groove Design
Proper seal design is a necessity for elastomer seals to perform reliably over the long term. Many of the instrument applications mentioned above use o-ring seals. The suggested compression for an elastomer o-ring seal to perform properly is typically a minimum of 16%, and a maximum of 30%. However, this range must also take into account the thermal expansion of an elastomer at elevated temperatures as well as any swell due to chemical exposure. Many of the elastomer seals used in instruments are small o-rings, which can create design issues. This is especially true for perfluoroelastomer parts which have a relatively high coefficient of thermal expansion (CTE). Fluoroelastomers have a lower CTE, making seal design easier at elevated temperatures.
Below is the second section of the white paper, diving into applications where the measurement is made in analytical laboratories which employ numerous solvents in a wide range of analyses and test equipment.
The final set of instrumentation is laboratory test equipment. As opposed to the laboratories in chemical plants, which often perform the same routine analyses on plant process streams, general analytical labs employ numerous solvents in a wide range of analyses and test equipment. As such, the ability of seals to resist a breadth of chemicals without degradation or leaching contaminants into a sample is of great importance. Although instrument seals are easily replaced in a laboratory environment, this operation still takes a technician time. It is always easier if the system can be flushed with a cleaning solvent and then be ready to run the next sample versus having to change out an elastomer seal due to incompatibility with a solvent.
Below is the first section of the white paper, diving into applications where the measurement is made at the process and the results then transmitted to a control system. This section will review the four types of in-line measurement devices, all involving slightly different elastomer sealing applications.
In-Line Process Applications
Flowmeters are used to measure the flow of liquid. In this section we will only consider the measurement of liquid flow in a closed piping system. Several examples of flow measurement devices include: flowmeters, Venturi tubes and orifice plates.
Note that these devices are “in-line” and require isolating the process line to remove and repair, or replace the measurement device. Shutting down a process to remove a device is time consuming, involves loss of production, and may require specific procedures to protect the operators and environment when a line is opened. All of these devices require seals to prevent leakage of the process to the environment and the elastomer seals should last the life of the flowmeter. For aggressive chemicals or high temperature applications, FKM or FFKM seals are an excellent choice. These products offer a long service life and resist deterioration in harsh environments.
The term instrumentation covers a wide variety of applications. In the broadest sense, instrumentation may be considered as any equipment used for measurements. This equipment may be in a process stream and include devices such as flowmeters, pressure gages, and inline probes. Data from these devices are used for process control. In automobiles, sensors are used for a variety of applications including measuring the exhaust stream to “tune” the engine to yield maximum performance. Analytical laboratory instruments such as chromatographs and flame ionization detectors are used to determine the composition of samples. Instruments are used in the medical industry for product analysis as well as analysis of blood and urine samples. Of course this is only a partial list of the many applications involving instruments.